Abstract
Before the advent of the innovative achromatic lens, it was recognised that it should be possible to obviate the problems of spherical and chromatic aberrations by having a long focal length objective lens. This stemmed from an empirical observation that the image quality of a lens—remembering they were all of spherical section and made of crown glass—was improved by a decrease in curvature. Taken to the extreme limit, a flat piece of glass has neither spherical nor chromatic errors, but also no capacity to focus closer than infinity. By having reduced curvature and therefore a long focal length, the aperture ratio (f) decreases. The f value is created by the simple expedient of dividing the focal length by the aperture. Thus, the best images at the time were generated by long focal length lenses with a resulting very small aperture ratio (confusingly designated by a large number). Aperture ratios of 1:150 were not unknown at this time and were an inevitable result of only being able to produce long focal length lenses of relatively small diameter.
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Wall, W. (2018). Aerial Refractor Telescopes and the Development of Reflectors. In: A History of Optical Telescopes in Astronomy. Historical & Cultural Astronomy. Springer, Cham. https://doi.org/10.1007/978-3-319-99088-0_4
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DOI: https://doi.org/10.1007/978-3-319-99088-0_4
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